Yielding Behavior of Tough Semicrystalline Hydrogels

Abstract

Supramolecular semicrystalline hydrogels\nare soft functional materials consisting of water-swollen hydrophilic\npolymer chains interconnected by hydrophobic segments forming lamellar\ncrystals. Although such hydrogels with high crystallinity are mechanically\nstrong, with elastic moduli and tensile strength of 80–300\nMPa and 4–7 MPa, respectively, they are brittle and rupture\nat a stretch of less than 20% without yielding. Here, we report that\nthe incorporation of a small amount of a weak hydrophobe into semicrystalline\nhydrogels significantly increases their toughness and stretchability\nwithout losing their high modulus and high strength. We design a highly\nentangled physical network based on poly­(<i>N</i>,<i>N</i>-dimethyl­acrylamide) (PDMA) chains containing <i>n</i>-octadecyl acrylate (C18A) and lauryl methacrylate (C12M)\nsegments with side chain lengths of 18 and 12 carbons, respectively.\nBy including 0.1–0.4 mol % C12M into the PDMA backbone containing\n30 mol % C18A segments, we were able to create more ordered and thinner\nlamellar crystals with a layered structure. Simultaneously, a brittle-to-ductile\ntransition was observed due to the appearance of necking behavior\nleading to 10-fold increase of toughness. The significant toughness\nimprovement upon incorporation of C12M into the semicrystalline hydrogels\ncould be explained with the appearance of active tie molecules under\nexternal force interconnecting the lamellar clusters. The hydrogels\nalso exhibit reversible tensile deformation induced by heating above\nthe melting temperature of crystalline domains.